If I did the math right (fingers crossed), for a minimum diameter three rotor rocket (three equilateral sides wrapped around an engine mount) versus a minimum diameter 4 rotor rocket (four equilateral sides wrapped around the same size engine mount), the 4 rotor version has .77 times the cross sectional area also (interestingly) .77 times the surface area of the 3 rotor version. Also assumes the same length of rocket.
Originally Posted by GrossApproximator
Results in some interesting compromises.
The 23% reduction in cross sectional diameter and 23% reduction in surface area should provide a higher apogee.
The reduction is surface area will also reflect a reduction in weight. While the rotor is only part of the weight of the rocket, the nose will also be smaller and lighter, and also the hinges. All these are proportional. Engine and engine mount will be unchanged. Also any "stick" running down the middle to connect the engine mount and the nose will probably be unchanged. So estimate about 2/3 of the weight of the rocket is variable, so perhaps a 15% reduction in weight. So that oughta help some too as far as apogee.
On the down side:
1. Will result in a 23% decrease in rotor surface area.
2. Will result in a 33% INCREASE in "lead edge length" on the rotors (going from 3 rotors of same length to 4 rotors of same length). I am NOT sure actually whether this is good or bad, but I think it's bad. I suspect this will inherently slow the rotor rotation speed.
3. Will need to go from 3 fins to 4 fins.
An unknown factor is what the effect of drag is between three 120 degree "corners" and four 90 degree "corners." Initially I had figured this would increase drag (more corners, more turbulence.) The Apogee Rockets article mentioned above however suggested that increase number of "shallower" corners is actually better. Even if theoretically true, some of this depends I believe on how tightly fitted the rotors are on launch. There is a potential for lots of little "interstices" between the rotors which would bump up drag. One of the goals of my design is to have the rotors really "tight" on boost, simulating a true square rocket tube rather than 4 balsa "slats" held togethe around a body tube.
One of the things that has always bugged me about most helicopter designs is the lack of "smoothness" between the nose cone and the rotor hinges. I've read that the nose cone creates a boundary layer or some sort of diversion of flow that reduces the drag causes by the position of the rotors, but I've never really been convinced. (Too bad I can't play with a wind tunnel like I did in school. Well, there are good things about not being a cadet anymore!) My pyramid nose cones I THINK might smooth some of this transition, although they are imperfect, probably better compared to the relatively sharp angulation between a true conical nose cone/body tube transtion vs. an Ogive or Elliptical cone/body tube transition.
Still gotta be better than the hub edge of rotor just sticking out.
One of many options when going from 3 to 4 rotors is to increase the length of the rocket proportionately to keep the same total rotor surface area. If done correctly, the cross sectional diameter is STILL reduced by 23%, the body surface area is constant, the mass is constant (okay, the central "stick" is a bit longer, but that's pretty minimal portion of the mass) and the rotor surface area remains constant. With a LONGER rocket I believe (???) you can probably use small fins as the CG will move forward a bit (isn't that why squatty rockets are unstable without a ton of nose weight?.) Again on the down side, You still need 4 versus 3 fins. Also, your "lead edge" length goes WAAAAY up, 33% just based on the additional 4rth rotor, and another 30% based on the additional length of the rocket/rotor. Again this is going to slow down your rotation quite a bit.
Something I really DON'T understand is the relationship between rotor rotation speed and rotor lift efficiency. Are my slower rotors "worse" just because the rotate slower?
Also begs the question (and I know I'm going to hit a nerve here), how much of the helicopter rotors effect on the rate of descent is due to rotational rotor "lift" versus simply airbrake recovery that just happens to be rotating? Note even with airbrake recover, the rotation is an advantage if it maintains the orientation of the "braking surface" perpendicular to the descent path. My Gyskelion (and I will emphasize that rocket is NOT a competition model as it is engine eject) is not airfoiled at all (hey, it's a sport model), in fact it is about as much Edmonds CiCi glider flat completely unsanded unfinished balsa as you can get (for the record, I really liked the CiCi.) Gyskelion gets about 60 seconds on a C6. Actually, since it is a sport model, I really don't WANT it to get any more, stupid thing goes a couple of hundred yards or more downwind with a 5 mph breeze!! Last time I launched it that puppy landed in the middle of a tennis match 400 yards away.
I built one helicopter rocket that for some reason (probably not enough angle on the rotors) didn't rotate at all. That puppy was weird, came down slowly, stable, smooth as silk. Touchdown speed was pretty similar to most parachute models, maybe even less.
Ooops, forgot, this is the competition page
Again, I really enjoy throwing these ideas out, not sure if they are going to pan out but it is fun to bounce things off the experts.
Also noticed there wasn't that much traffic on the competition section anyway
It is amazing what you can do when you don't have a choice.
Smart people learn from their mistakes.
REALLY SMART PEOPLE learn from OTHERS' mistakes.